1. Field of the Invention
The invention relates generally to the field of optical recording systems and more particularly to apparatus for detecting and decoding sector marks therefrom.
2. Brief Description of the Prior Art
Sector marks generally comprise areas of the optical surface having information recorded thereon which violates one or more rules by which the data is recorded. In optical recording systems of a type employing prerecorded clock signals on pregrooved disks, the sector marks have either had a gap in which could be detected by a low pass filter, or by threshold detectors and prerecorded data which was detected input to a shift register, or a combination of the two. When the detected pattern in the shift register corresponded to the pattern of a sector mark, a sync pulse was issued indicative of such detection.
It is not desirable to use a gap as a part of a sector mark in a device having a prerecorded clock in a pregrooved disk. This entails loss of both disk clock information and disk servo system tracking information. Loss of clock information can cause a phase lock loop, locked to the prerecorded clock, to drift, even if marginally, to degrade system performance. The same is true of servo systems when tracking information is lost.
Secondly, it is not desirable to use threshold detection in combination with shift register decoding. In optical systems, it is very difficult to set such a threshold. The optical recorder reading the information from the optical disk does so conventionally by means of a laser operated at read power. The beam reflects from the disk, and the drop in reflection normally indicates the presence of a hole or a prerecorded mark. Because reflected spot density distributions have a Gaussian shape, the hole associated power of the reflected beam (the hole associated power means the inverse of the reflected power from the disk) spreads a significant distance beyond the boundaries of the holes themselves. Indeed, the hole power present at the center of the next possible position of a hole or a prerecorded mark in closely spaced systems may be significant. Therefore, one cannot rely on threshold detectors to detect the presence of a hole or a prerecorded mark, because the threshold may be reached due to a hole or a prerecorded mark at an earlier or later position, or a combination of the two. Additionally, hole sizes vary widely causing correspondingly wide variations in the amplitude of the signals indicative of the presence of holes. As well, prerecorded material, such as sector marks, cause drops in reflection far less than that caused by burned holes. This necessitates a variation in the threshold level depending on the type of material being read. Moreover, other system variables negative the use of threshold detection. Among these are reflectivity variations from disk to disk and across a single disk, variations in laser power levels, optical efficiency, sense diode coupling errors, circuit tolerances, etc. For these reasons, an alternative method of detection of sector marks is desirable. The method employed by the preferred is that of differential detection.